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Greek and Roman astronomy
Greek astronomy began during the eighth century BCE. The earliest surviving sundial of antiquity dates from that century. The Greek poet Hesiod, in Works and Days, wrote that the wise husbandman knows when to engage in agricultural tasks according to the appearance and movements of the Pleiades, Sirius, and Orion. Homer's poems identified similar celestial phenomena as well. The Greeks, like the Babylonians and Egyptians, brought their mythology to the heavens, naming groups of stars according to great heroes, legendary figures, and animals. The fixed stars formed the background for the wandering heavenly bodies, the planets, which move on the same plane in the band of the zodiac. Early Greek scientists developed the idea of the heavens as a great sphere arching over the Earth. Astronomers observed the continually changing horizon and tracked the appearance of stars in the direction of the Earth's rotation. Greek astronomers recognized the eastward rotation of the Earth; the ecliptic, which is the path of the sun across the horizon over the course of a year; the zodiac, the path of the sun and planets on the ecliptic; and scores of constellations, which they named according to their own myths and legends.
Ionian Greeks such as Thales and Pythagoras formed the first sophisticated school of Greek astronomy. Pythagoras reputedly developed the idea of separate perfect spheres that define the orbits of the planets, moon, and Earth. Planets rubbing against each other cause a sound, the harmony of the spheres. Pythagoras or his followers realized that the moon reflects the light of the sun, and that the varying shapes of the moon prove its sphericity, which led to the deduction of the Earth's similarity in this regard. Lunar eclipses as well reveal the curved Earth surface. Philolaus, a follower of Pythagoras, hypothesized the moving Earth. He conceived that the Earth cannot be the center of the universe, but rather there must be a central fire around which all the celestial bodies orbit. The sun, moon, and seven planets make nine spheres orbiting around the central fire. Nine is not such a significant number, however, compared to 10 (the sum of 1, 2, 3, and 4). There must be a 10th sphere, which Philolaus imagined to be the antichthon, the "counter Earth," which always stays hidden from humans—being precisely on the other side of the sun, it is never seen.
The Pythagorean concept of the central fire was a heliocentric theory based upon philosophy rather than observations and mathematics. Aristotle, the leading scientist of his age, rejected such a scheme for the geocentric universe. Besides the obvious experience of the sun passing daily from east to west, which according to common sense reveals that it moves while the Earth is still, Aristotle argued that for the Earth to be orbiting the sun the stars would pass, rise, and set like the sun and moon, which is not the case. Aristotle was too much the practical observer to realize that the distance of the stars from the Earth prevents any significant movement in the night sky. In other words, there is very little parallax, stellar movement respective to the Earth, a phenomenon that escaped Aristotle.
An understanding of parallax did not, however, escape Aristarchus of Samos, the first astronomer to argue for the heliocentric universe. Aristarchus assumed that the stars are at such a distance from the Earth that their movements respective to the Earth are scarcely noticed. He used geometry to try to discover the relative distances and sizes of the sun and moon to each other and to the Earth. His estimates were erroneous yet still revolutionary in the attempt.
Hipparchus of Bithynia was a greater astronomer than Aristarchus, even though Hipparchus advocated the geocentric universe. Hipparchus worked at Alexandria and Rhodes—at the latter city creating something like an astronomical research center in which he catalogued the stars according to brightness, which indicated to him relative distance from the Earth and location. He ended up with a star chart of about 850 stars. Hipparchus made more accurate measurements than Aristarchus did of the distance of the moon and sun from the Earth. He discovered that the Earth's axis changes over time. To try to account for the orbit of the sun around the Earth, he developed the idea of the eccentric, which Ptolemy of Alexandria would expand upon.
Other Hellenistic astronomers included Callippus of Cyzicus, a student of Eudoxus and Aristotle, who studied the theory of concentric spheres of heavens, theorizing one for each planet. He charted and wrote on the rising and the setting of stars. Callippus is remembered for his On the System of the Planets. His contemporary, Heraclides of Pontus, developed a system combining geocentric and heliocentric systems, in which the sun and moon and outer planets orbit Earth, but Mercury and Venus orbit the sun. Heraclides believed that the Earth rotates on its axis every 24 hours. Autolycus of Pitane (ca. 300 BCE) wrote treatises on the heavenly spheres and the movement of the stars. Aratus of Soli penned Phaenomena, a treatise in verse on constellations and stars. Alexandrian astronomers included Menelaus, working in the late first century CE, who used trigonometry and the geometry of the sphere; Hypsicles, a second-century CE mathematician and astronomer who calculated the length of each day at Alexandria and the extent of the zodiac; and Ptolemy, the greatest astronomer of the Roman Principate.
Ptolemy's accomplishment was a reasonable explanation for the problem of the wandering planets, retrograde motion, which occurs when a planet farther from the sun than Earth is "passed" by Earth in their respective orbits. The position of the planet in the night sky alters—seemingly reverses and then resumes its previous course. Retrograde motion of outer planets was confusing to the ancients until Ptolemy came up with his explanation of epicycles, equants, eccentrics, and deferents. Ptolemy's scheme was, of course, based on an Earth-centered universe. He imagined that as planets orbit the Earth they also orbit about a point in their Earth-orbit called the epicycle. The deferent is the orbital pattern around the Earth. The eccentric is the center point of the orbital scheme. Earth is not directly in the center of the orbiting planet's deferent. The equant is the true point about which the planet orbits. This confusing scheme did not reflect reality in the least. Ptolemy was not concerned about reality; he was concerned about a mathematical scheme that could predict the motions of the planets, and in this goal he succeeded brilliantly.
Pliny the Elder's Natural History provides a good summary of how Roman scientists absorbed Greek astronomy. Pliny assumed that the universe is godlike, possibly infinite, and unknowable. The spherical Earth, revolving at a tremendous rate of speed, is at the center of the universe. For Pliny, the universe is harmonious and regular, the product of a divine mind utterly beyond human understanding. He thought that perhaps the harmony of the planets produces a beautiful sound, although humans cannot hear it. The planets are distant to the Earth in the sense of their respective elevation approaching the starry vault. Each revolves around the Earth in a set period; each has a certain character that gives a particular identity. Pliny also wrote that Venus and Mercury, unlike Mars, Jupiter, and Saturn, rarely extend much above the horizon, in morning or in evening. Venus's light is third only to the sun and moon—and thus like those two bodies has a clear impact on human events. The moon is easily as large as the Earth, though the sun dwarfs them both. Pliny discussed the phases of the moon, the solstices and equinoxes of the sun, and the varied forms of eclipse. He discussed comets and their influence on Earth events. Indeed, Pliny discussed meteorology in light of astronomy, believing that storms, thunderbolts, climatic change, and the like are influenced by the sun, moon, planets, and stars. He was willing to believe in astronomical portents of changes in human affairs, yet such credulity was countered by some interesting discussions, such as his account of rainbows as being the contact of sunlight with clouds, which is a close approximation to the modern view that molecules of light are refracted by water vapor.
Other astronomers of the Roman Empire included Theodosius of Tripolis, an astrologer and astronomer of first century CE; Posidonius of Rhodes, who influenced Geminus of Rhodes, the latter of whom wrote the treatise Introduction of Astronomy; Heliodorus of Alexandria, late fourth century CE, who wrote a commentary on Ptolemy; and Manilius, first century CE, who wrote a poem, the Astronomica.
For most periods in the history of science, astrology and astronomy were scarcely distinct studies. In the ancient world, for example, astrology provided a big impetus to the development of astronomy, as astrologers wanted to know the positions of planets, the moon, and the sun at various times. Astrologers believed that the movement and position of the planets had a determining effect on the future. Theophrastus the Peripatetic scientist thought the Chaldeans (Babylonians) and Herodotus thought the Egyptians could predict the future through astrology, and Ptolemy wrote a treatise on astrology, the Tetrabiblos.